Wow, I need to write fast because I'm impressed by the few reviews I've read.
A very beautiful film, delightful how the creators were able to "get close" to dilemmas, stresses, joys, young people at the stage of searching for love, searching for themselves, learning each other in relationships.

Na-Bi
What do we know about her ?: You can't see dad in her life at all,.Her mom used to hang out with a lot of hopeless guys. Na Bi, looking at her mother's love failures, she decides:
"When I was little. I promised myself I would never date while my mom". Of course, you can make a lot of decisions, but if you do not have a model of a happy relationship in your parents (guardians) as a child, you do not know what to do. Already in the first episode we see that Na Bi chose a guy in the style of her mom's guys. I don't blame her for that, I know it is, there is an automaton that is stronger than the resolutions. If in childhood, in the family home, love is mixed with violence, when we are looking for a partner, then the emotional matrix embedded in childhood pushes us into the arms of the torturer. Unfortunately, Na Bi's mum is still in the same place when dealing with guys, and farther and farther away from Na-bi. They see each other very rarely and their meetings are sad - there is no love for Na Bi.

Na Bi has a loving, warm, wise aunt, (great, great role, I'm delighted with this actress in this role).
When Na Bi is angry with herself, behind having sex with Jae Eon and she says, "There was nothing between us. I don't know why I did that."
S1: 06 24:57 My aunt said, "Hey, there is always something between people
Butterflies don't always eat honey. Insects and poop are all nutrients for them too.
Everything you're going through in your life right now, are your nutrients. So what's there to fear? "
Aunt is attentive and also sees clearly that Na Bi's relationship with Jae Eon is big. Na Bi, he listens to Auntie in conversations, but it seems that he hears nothing .

Jae Eon
What do we know about him? You don't see dad in his life at all. He has also lived without his mother since the age of 10, and it seems he rarely sees her. They meet at the hotel (sad) because mum has such a job, she is a dancer.
There are no relationship patterns, and like Na Bi, he has patterns of his own making. What he thinks about relationships we learn from his conversation with Kyu Hyun.
Kyu Hyun says what he expects in a relationship: "Certainly about the relationship and sincerity toward me".
Jae Eon said, "That's unrealistic. As soon as you have certainty, that relationship becomes weak".
He is in an open relationship with Seol Ah (they both know they have other partners too - such a deal) He is dating a lot of girls.
Oh, one more thing, his apartment is black, cold, sad, gloomy, dark, and it's not a minimalist, artistic style at all, and certainly not for lack of money.
Actually, we meet Jae Eon when he meets his mother.
When Jae Eon told her mom about Na-Be: "Just someone bothersome".
Of course, Mom knows her baby and says to Jae-Eon:
"You known what I regret the most? Not knowing my own feelings. No, pretending not to know. Who would love someone, who doesn't even take care of themselves?" (and all clear)

Two emotional cripples come together who have created invented rules of relationships in relationships.
It is obvious to everyone from the outside that it sparkles that they are made for each other, only they, each of them individually, cannot name their feelings and talk about them ..
Jae Eon in a relationship with Na Bii.
Cordial, attentive to needs, helpful in difficult and very difficult and embarrassing situations (S1: O2 35:21). They are not a couple yet, but they are handcrafted by a sun-butterfly catcher for Na Bi (he could have bought something because with money, no problem).
When he thought that Na Bi was so sick that she needed help and the door was closed, he called the police and an ambulance, he was terrified that he would not be able to help. He looks after Na Bi, at night he goes to the store, buys food and medication, she sleeps sitting with the patient all night holding her hand.

Sex and eroticism: Jae-eon signals her needs but never pressures, withdraws when Na Bi makes it clear that she doesn't want to. He always makes sure Na Bi wants what he wants.
When Jae Eon came with a book to study anatomy, he was honestly surprised to find alcohol on the table, but it seems Na Bi was thinking about sex.
In sex, he is sensitive, gentle, attentive and takes his time to satisfy his needs .. I think that there is a scene where he washes dishes and says that he will also do laundry, it's like this place on earth at Na Bi becomes his home and takes care of it.

Na Bi in relation to Jae Eon.
Already at the first kiss (she kisses him) she gives a signal that she does not mind Jae Eon's relationship with other women. First, she rubs another girl's lipstick from his lips and kisses him in a moment.
Constantly and still does not know how to name his feelings, love does not exist - such a slogan, is not sure what to do, blames himself for the feelings he finds in himself, blames himself for sex, sets himself up as a victim and stays in this role until the end, distrustful, suspicious. He does not talk about his fears, doubts with him, but with himself. He makes a deal with Jae-Eon, but constantly criticizes Jae Eon and looks for flaws in him. She becomes a torturer for herself.
When I look at her role in this relationship, she takes, she takes, but I don't see what she gives. Similarly in relationships with other guys, he takes, he takes, I don't see what he gives.

Just as she doesn't hear what Auntie is saying to her, she doesn't see Jae Eon changing koro for her step by step.
When Na Bi in S1 O7 53:43 says, "I, m the one who made a mistake", I was hoping she was starting to go her own way too, but you have to watch the end to find out.

Throughout the series, we have the opportunity to observe this couple and participate in changes in the relationship. For me, what is fascinating and I recommend it.